This invention relates generally to CATV scrambling systems and specifically to CATV scrambling systems that incorporate pole moounted scrambling units.
A CATV system includes a cable station, commonly referred to as a head end, that supplies a plurality of television (TV) signals or "channels" over a network of cables to a group of subscriber locations for a fee. Most cable systems offer subscribers different levels of programs or service. For example, there s usually a basic television service and, for an extra fee, the subscriber may receive a movie channel, a sport channel or other special programming events.
A difficulty exists in most cable systems that use "in-home" converter/decoders, due to their incompatibility with video cassette recorders (VCR's) and full featured television receivers. This is because the output of the converter/decoder is restricted to VHF channel 3 or VHF channel 4 and essentially nullifies the multi-channel tuners in the VCR and TV. So-called "on the pole" systems circumvent this difficulty by passing all of the authorized television channels to the subscriber so that his VCR and television receiver are fully funtional. Pole systems are generally categorized as negative or positive. Negative systems individually trap out the premium channels from the channels passed to the subscriber. This requires an expensive trap for every premium channel and the traps themselves are quite temperature sensitive. In short, it is difficult and expensive to built reliable traps for the temperature range in an outdoor environment. Positive systems, on the other hand, insert an interfering signal on each premium channel, which is turned off for an authorized channel. Here again, the oscillators required to generate the interfering signals are sensitive to temperature variation and relatively expensive to manufacture. In such systems, a change in service level may require accessing equipment on the pole to add or change a trap, unless expensive automoatic equipment is used. Also, power for the pole systems is supplied by the cable head end which imposes a significant operating cost burden on the cable operator.
The system of the present invention is a "hybrid" and involves a pole mounted scrambling unit and a subscriber location unit. Critical components are used in the subscriber unit that operates in a relatively benign environment, whereas the low power consumption pole unit uses low cost temperature insensitive components. While each subscriber location is equipped with a pole unit, only subscriber locations for which premium channels are authorized are equipped with a subscriber unit. Indeed, the subscriber unit need not be accessible to the subscriber since it is controlled from the head end. A subscriber unit, when present, also supplies operating power for the pole unit.